1. Field of the Invention
The present invention relates to a method for chemically treating ceramic substrates prior to metallization thereof in order to provide at least one electrical circuit thereon and, in particular, to treating ceramic substrates prior to metallization with an alkali hydroxide melt.
2. Background of the Art
Ceramic substrates are metallized in order to change, improve, or supplement the decorative and/or functional characteristics of the ceramic objects, such characteristics including electrical and/or thermal conductivity, resistance to corrosion, and resistance to wear. For whatever reason metallized, the adhesion of the metal coating on the ceramic material is always of particular significance. Very generally speaking, the adhesion of the layers is produced either by a relatively weak interaction between the material of the layer and the substrate (so-called Van der Waals forces), by chemical bonding, by mechanical anchoring, or by a combination of these contributory factors. The contribution made to layer adhesion by these individual factors depends to a special degree on the type of pretreatment received by the ceramic substrate and on the metallization system selected.
Several processes are known for the metallization of ceramic substrates, including thick film and thin film techniques. Ceramic plates for circuit boards and the like may be provided with thick film circuits by thick film techniques, such as, the well known screen-printing of noble metal pastes thereon followed by a burn-in. Circuit board art and the like also include techniques for providing so-called thin film circuits by way of vacuum vapor deposition or cathode sputtering. These processes, however, are limited to certain applications, in part because they are expensive and because of various process-specific unfavorable characteristics of the metallizations produced thereby. For example, the thin film technique for producing gold or copper layers, requires the use of adhesion-promoting intermediate layers of chromium, titanium or molybdenum, which can behave as impurity. The thick film technique is frequently characterized by a considerable reduction in the conductivity of the conductor materials because of embedded glass components and, moreover, because of the screen-printing step, the path width of the conductor materials cannot be made significantly smaller than about 200 microns.
In the past, many attempts have been made to apply conductors of a pure metal, particularly copper, directly to ceramic surfaces. Pure metal conductors have greater conductivity and therefore require a smaller conductor path cross-section. One such process is based on electroless deposition, i.e., the currentless chemical precipitation, of copper on a ceramic surface previously covered with catalytically active nuclei capable of catalyzing the electroless precipitation of metals thereon from a chemical bath. In this prior art process, the adhesion of the copper layers is generally insufficient or the copper layers are too faulty to be suitable for the reliable production of conductor path patterns having fine structural configurations, because of pinholes and/or a tendency to blister and peel.